Coated ceramic discharge vessel for improved gas tightness

ABSTRACT

The present invention relates to a discharge vessel ( 1 ) with at least one end part ( 2 ) and a discharge cavity ( 3 ), whereby the discharge vessel ( 1 ) is at least partly coated with a coating layer ( 4 ), and whereby the gas-tight bonding of the coating layer to the discharge vessel ( 1 ), to a sealant ( 5 ) and/or to an end closure member ( 9 ) is stronger compared to the direct gas-tight bonding of said sealant ( 5 ) to said end closure member ( 9 ) and/or discharge vessel ( 1 ).

The present invention relates to a high-pressure discharge lamp, such asfor instance an automotive lamp used for head lighting applications,comprising a ceramic discharge vessel, which encloses a dischargecavity, and at least one end opening with at least one coating layerdeposited onto it, and preferably a feed-though opening, gas tightclosed by an end closure device. Said end closure device comprises atleast an end closure member and connection means. More particularly, theinvention relates to the design of said discharge vessel and ways toimprove its lifetime.

Discharge vessels in high-pressure discharge lamps and relatedmanufacturing processes are known from a prior art. Nevertheless, it isstill necessary to provide a design of said high-pressure dischargevessel addressing the drawbacks known from said prior art as mentionedlater on. Due to said high pressure filling, gas tight closing saidhigh-pressure discharge lamp discharge vessel causes several problems.Heating said discharge vessel for gas tight sealing leads said internalfilling to expand or evaporate. As a result, filling gas expansioncauses a bad quality seal, and filling salts evaporation givesunexpected lamp characteristics. Said seal is then characterized in thatit ends up with an irreproducible length, since expanding gas tends topush it outwards from said discharge vessel. Moreover said seal willcontain defects, such as gas bubbles, leading to cracks, which weakensthe seal mechanical strength, leading to leakage.

In order to prevent the expansion or evaporation of said filling,several attempts to find alternative sealing processes and designs havebeen made.

WO 00/67294 describes a high-pressure discharge lamp, more precisely ametal halide one, with a very small, very high-pressure filled vessel,surrounded by a gas filled outer bulb.

Said lamp has the advantage of having a discharge vessel with verycompact dimensions, which makes it highly suitable for head lightingapplications in motor vehicles. Thanks to the discharge vessel internaldiameter, small compared to the electrode spacing, the discharge arc issufficiently straight, and its light emitting surface sufficientlysharply limited, so that it can be used as a light source in anautomotive headlamp, especially in a headlamp with a complex-shapereflector.

The drawbacks of the known lamp are however a relative loss of theinitial filling while heating up said lamp's discharge vessel asgas-tight closing it. It leads to a wrong colour point setting and tocolour instability. Drawbacks also comprise an irreproducible initialsealing ceramic length while gas tight closing said discharge vessel, asealing ceramic cracking behaviour within the high lamp-operatingtemperature range, which leads to a leaky seal. Furthermore saiddischarge vessel end construction design comprise a wide clearance,between said feed-through outer surface and the ceramic plug inner wall,which leads to colour instability. These drawbacks are caused by thecurrent sealing process, or are related to the current sealing design.Said process is actually heating far too much surface of said filleddischarge vessel, and said design is leaving far too much clearancebetween said feed-through and said ceramic plug. Both the feed-throughand the ceramic plug are furthermore made of inappropriate thermomechanically matching materials.

U.S. Pat. No. 6,194,832 B1 describes a metal halide discharge vessel,wherein a plug—having at least four, and preferably six or more, axiallyarranged layers or strata of a cermet, in which the metal content of therespective layers or strata increases from the layer or stratum closestto the discharge space of the vessel outwards—is used to close off saidmetal halide discharge vessel tubular ends. The innermost layer orstratum is directly sintered to the ceramic discharge vessel, typicallyof aluminum oxide, whereas the outermost layer or stratum has a metalcontent of such an extend that it can be welded, and is welded to ametallic or cermet feed-through part projecting to a central openingthrough the respective layers or strata of the plug. The outermost layerof the plug, preferably, has at least 50%, by volume, of metal,preferably of the same material as the feed-through part, and may evenbe entirely of metal, to ensure a tight, easily made weld connection.The weld can be made, for example, by laser welding. It turns out,however, that since the innermost layer of the protruded plug is notconnected to the feed through, and has a low metal content that does notmatch the coefficient of expansion of the feed-through, it leads bydesign either to the formation of a crevice if there is no contact, orto stress build up in the plug if there is contact, thus to a loose fitbetween the plug and the feed-through very close to the vessel, i.e. toa crevice as well. Obviously salts will creep in said generated loosefit, which will lead to colour instability of the lamp, and in parallelto that, to the extension of said loose fit outwards along the feedthough. For very compact automotive discharge vessel operating atrelatively high temperatures, moreover going through a very high numberof switches, both drawbacks are critical and lead to further leakage.

It is an object of the present invention to improve the gas-tightconnection of the end closure device to the discharge vessel, in orderto prevent a loose fit between said parts from forming, whereincorrosive salts filing could condensate.

This issue is addressed by partly coating a discharge vessel with atleast one end part and a discharge cavity, whereby at least one of saidcoating layer is deposited and gas-tight connected between an end partof said discharge vessel and a sealant and/or between a sealant and anend closure member.

Said layer improves the binding of the connection means, whereby thelayer provides higher adhesive strength between the discharge vessel andthe connection means, and/or between the connection means and the endclosure device, compared to the adhesive strength between the dischargevessel and the end closure device.

Said end closure member is usually connected to said discharge vesselwith a sealant in order to achieve a strong bonding between said parts.Due to different states of surface, and different expansion coefficientsbetween said end closure member and said discharge vessel, the gas tightbinding is weakened after the operated lamp has run through a series ofthermal cycles. In order to improve the gas tight bond and provide amore reliable burner, at least one coating layer is deposited onto atleast a part of the end parts of the discharge vessel.

This coating layer is applied onto the discharge vessel in its greenstate before the firing step of the discharge vessel sintering process.

Advantageously, at least one end part of the discharge vessel is atleast partly coated with a layer. Said layer improves the binding of theconnection means, whereby the layer provides higher adhesive strengthbetween the discharge vessel and the connection means, and/or betweenthe connection means and the end closure device, compared to theadhesive strength between the discharge vessel and the end closuredevice. Most preferably, a first layer is located between the dischargevessel and the sealant, and a second layer between the sealant and theend closure member.

Since the gas-tight bonding of the end-closure member to the dischargevessel is improved thanks to said coated layers, loose fit seals,crevices or small cracks are reduced.

In order to prevent materials from cracking, coating layers, connectionmeans, as well as end closure members should be made of materialscharacterized in that they have thermal expansion coefficients matchingwith one another and matching with the one of the discharge vessel.

In a preferred embodiment of the present invention, any coatingmaterial, which expansion coefficient α (T) matches the expansioncoefficient of a polycrystalline alumina discharge vessel of about8·10⁻⁶ K⁻¹ can also be suitable used for the present invention. Suchmaterials are characterized in that their expansion coefficients α (T)lie in the following range: 4·10⁻⁶ K⁻¹≦α (T)≦12·10⁻⁶ K⁻¹ fortemperatures T lying in the following range: 298 K≦T≦2174 K. Mostpreferably, said coating material should have an expansion coefficientas close as the one of the discharge vessel as possible in order toprevent stress build up in the coating that would lead to cracks in thelayer.

Therefore, the coating layer is preferably of a material selected fromthe group comprising for instance W, Mo, and/or Pt. Said materials havea thermal expansion coefficient lying in the range of 4·10⁻⁶ K⁻¹ to12·10⁻⁶ K⁻¹ and are corrosion resistant towards a typical dischargevessel metal halide filling.

Sealants according to the present invention are means for gas-tightconnecting at least two parts, preferably for gas-tight connecting anend closure member to a coating layer deposited onto a discharge vesselof a high-pressure burner.

Connection means according to the present invention are also means forgas-tight connecting at least two parts such as a feed-through with anend closure member.

Sealants as well as connection means in the sense of the inventioncomprise materials that are needed for welding, laser welding,resistance welding, soldering, brazing, bonding with adhesive materials,primary shaping, sintering, sealing or any combination thereof.

The discharge vessel comprises end parts and a discharge cavity. The endpart is preferably in the form of a protruded plug. Said dischargevessel is usually closed by end closure devices, coated with at leastone layer, whereby said end closure devices are gas-tight connected tothe end parts of the discharge vessel in order to provide a gas-tighthigh-pressure burner.

In a preferred embodiment of the present invention a burner comprises atleast one end closure device, comprising at least one connection means,gas tight connecting the feed-through to the discharge vessel.

The end closure device is gas-tight connected to the discharge vesselwith at least one coating layer and at least one sealant.

It has been found that corrosion problems, caused by a crevice or cracksin the coating layer, seal and/or connection means between the dischargevessel and the end closure member, can be attenuated or avoided when theouter cross-section of the end closure device feed-through-opening is≧than the inner cross-section of the end closure devicefeed-through-opening. Such a feed-through-opening geometry enables a gastight connection of the feed-through within the end closure member,located at the innermost feed-through-opening section of the end closuremember facing the discharge vessel.

The end closure member feed-through entry-opening cross-section ispreferably designed to be larger than the end closure memberfeed-through exit-opening cross-section. Furthermore, said crosssections have preferably the same geometry. The feed-throughcross-section varies along the burner's main symmetry axis. The nearerthe feed-through-opening cross-section to the discharge cavity is, thesmaller. Connection means could then be located ideally directly at thefeed-through exit opening.

The opening is preferably a feed-through opening. By filling thedischarge vessel through a feed-through opening, and then gas tightclosing the feed-through opening, the thermal influence caused by theconnecting process is lower than the thermal impact caused by acomparable closing process of the end closure device onto the endopenings of the discharge vessel. Indeed, closing the feed throughopening requires a local and very quick heating. Furthermore, thanks tothe relatively high speed and local implementation of the gas tightconnecting process, the coating layers used to improve the bondingbetween the discharge vessel and the end closure device will not bedamaged.

According to the present invention, a crevice is the space between thegas tight sealed feed-through, and the part in which said feed-throughis arranged and sealed. Another definition of a crevice is the remainingspace of the feed-through opening, after the feed-through is arrangedinto said feed-through opening and gas tight sealed. More precisely, acrevice is the volume remaining from subtracting the feed-through partvolume from the feed-through opening volume. From the feed-throughopening volume is actually also subtracted the volume of the connectionmeans after the feed-through connecting process has been performed.

The outer form of end closure members according to the present inventionhas preferably the shape of a cork, a disk, a plug or an end cap.

The end closure member has a shape fitting to the end part of thedischarge vessel. Said shape depends on the location where the endclosure member is mounted. The end closure member can be inserted intothe end opening of the end part. In such a case, the end closure membercan have the form of a plug. The end closure member can be arranged sothat it contacts the end opening outer end part. In such a case, the endclosure member can have the form of a disc or of an end cap. Preferably,the cap can at least partly surround the end opening outer end part.Indeed, the end closure member can be advantageously located partlyinside said end opening and partly outside. In such a case, the endclosure member can have the form of a cork.

The end-closure device materials should have a thermal expansioncoefficient matching the one of the discharge vessel, so that no stressor crack builds up during the sealing process and the thermal cycles ofoperating burner later on. Thus, the end closure device, preferably endclosure member and/or connection means, is made of a metal, preferablyMo, a coated metal, preferably Ta coated with Mo or Al₂O₃, a metalalloy, preferably an inter-metallic such as Mo₃Al, of a cermet, and/orof a ceramic, preferably Al₂O₃.

The end closure member through-going feed-through opening cross-sectioncan have any suitable form; preferably said cross-section has theprofile of a cone, a parabola, a hyperbola, an ellipse, a hemisphere, aY-like profile, an O-like profile, a T-like profile or a X-like profile.

The sealant, connecting the end closure member to the coating layer, isa material comprising metal, metal alloy, and/or ceramic.

Should the end closure device be made of a cermet material, it wouldpreferably be a functionally graded material. A suitable cermet materialused according to the present invention has a substantially continuousgradient of at least compounds A and B, whereby the concentration ofmaterial compound A substantially increases in the same degree, in thatthe concentration of material compound B decreases. The concentrationgradient can preferably be described with any linear or non-linearfunction.

The cermet material comprising a gradient of at least compounds A and Bis characterized in that it has an outer layer, in which theconcentration of material compounds A and B are constant.

Said layer can have a thickness from 0 to 500 μm, preferably from 0 to50 μm and most preferably from 0 to 5 μm.

The compound A can be Al₂O₃ and the compound B can be Mo. Othercompounds can be mixed additionally to A and B in the same graded, or inan ungraded, manner.

In a preferred embodiment of the present invention, a gas-tighthigh-pressure burner comprises at least one of said end closure memberswith at least one feed-through.

Preferably, the gas-tight high-pressure burner coated partly with atleast one layer comprises at least one end closure member with at leastone feed-through, preferably the end closure member has at least onethrough-going feed-through opening, whereby the cross-sectional area ofthe feed-through opening varies in longitudinal direction of the endclosure member.

A further aspect of the present invention is to provide a lampcomprising said gas tight high-pressure burner. The lamp comprising saidburner is preferably arranged in a headlamp. Such headlamps arepreferably used in the automotive sector, especially in the carindustry, but are not limited to this use only.

Another aspect of the present invention is to provide a method ofmanufacturing a gas tight high-pressure burner, comprising at least oneend closure device, at least two feed-through parts, and at least onepartly coated discharge vessel, with at least one end opening, wherebysaid method comprises the following steps:

-   -   i) Filling said discharge vessel with an ionisable filling        through at least one opening, and    -   ii) Closing said opening by arranging a feed-through therein,        followed by gas tight connecting said feed-through to the end        closure device and/or to the discharge vessel, whereby a gas        tight high-pressure burner is obtained.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

FIG. 1 shows a longitudinal cross-section of a gas-tight high-pressureburner with a series of coating layers.

FIG. 2 shows a longitudinal cross-section of a gas-tight high-pressureburner with two series of coating layers.

FIG. 3 shows in detail a longitudinal cross-section of a coated end pluggas tight connected to an end part.

FIG. 1 shows a discharge vessel 1 with tubular end parts 2 and adischarge cavity 3, coated by a coating layer 4. The coating layer 4covers the surface of said tubular end parts 2. Said coating layer 4 islocated between the discharge vessel 1 and a sealant 5. In order toobtain a gas-tight high-pressure burner 6, said discharge vessel 1 isclosed by an end closure device 7 and a feed-through 8. The end closuredevice 7, gas-tight connected to the discharge vessel 1 by said sealant5 and said coating layer 4, comprises an end closure member 9 with afeed-through opening susceptible to arrange said feed-through 8 therein.Said feed-through 8 is gas-tight connected to the end closure member 9by connection means 10.

FIG. 2 shows a discharge vessel 1 similar to the discharge vesselpreviously described in FIG. 1. The discharge vessel 1 according to FIG.2 has a first coating layer 4 a and one additional second coating layer4 b. The additional second coating layer 4 b is located between the endclosure member 9 and the sealant 5 of the end closure device 7.

FIG. 3 shows an end closure member 9, more precisely an end plug with anend opening, gas tight connected to the discharge vessel 1 of a gastight high-pressure burner. A feed through 8 with an electrode isarranged into the end opening of the end plug, and is gas tight sealedto said end plug by a sealant 5. The end plug is made of a cermet,preferably of a functionally graded cermet. At its outer surface, facingthe discharge vessel 1, the end plug is coated with a first ceramiccoating layer 4 a, in order to improve its bond with said ceramicdischarge vessel 1. At its inner surface, facing the sealant 5, the endplug is coated with a second metallic coating layer 4 b, in order toimprove its bond with said metallic sealant 5.

LIST OF REFERENCE NUMBERS

-   1 Discharge vessel-   2 End part-   3 Discharge cavity-   4 Coating layer-   4 a First coating layer-   4 b Second coating layer-   5 Sealant-   6 Gas-tight high-pressure burner-   7 End closure device-   8 Feed-through-   9 End closure member-   10 Connection means

1. Discharge vessel (1) with at least one end part (2) and a dischargecavity (3), characterized in, that at least one coating layer (4) islocated and gas-tight connected between an end part (2) of saiddischarge vessel (1) and a sealant (S) and/or between a sealant (5) andan end closure member (9).
 2. Discharge vessel (1) according to claim 1,characterized in, that the gas-tight bonding of the coating layer (4) tothe discharge vessel (1), to a sealant (5), and/or to an end closuremember (9) is stronger compared to the direct gas-tight bonding of saidsealant (5) to said end closure member (9) and/or discharge vessel (1).3. Discharge vessel (1) according to claims 1 to 2, characterized in,that the coating layer (4) has an expansion coefficient in the rangebetween 4·10⁻⁶ K⁻¹ and 12·10⁻⁶K⁻¹.
 4. Discharge vessel (1) according toclaims 1 to 3, characterized in, that the coating layer (4) ischemically resistant towards oxides and iodides.
 5. Discharge vessel (1)according to claims 1 to 4, characterized in, that the coating layer (4)is of a material selected from the group comprising at least W, Mo,and/or Pt.
 6. Discharge vessel (1) according to claims 1 to 5,characterized in, that the coating layer (4) covers at least the endparts (2) of the discharge vessel (1) of the end closure device (7). 7.Gas-tight high-pressure burner (6) with coating layer (4) comprising atleast one discharge vessel (1) according to claims 1 to 6 and at leastone end closure device (7) and at least one feed-through (8). 8.Gas-tight high-pressure burner (6) according to claim 7 comprising atleast one end closure member (9) with at least one feed-through (8),preferably the end closure member (9) has at least one through-goingfeed-through opening, whereby the feed-through opening cross-sectionvaries along the end closure member (9) longitudinal axis.
 9. Lamp,comprising at least one gas-tight high-pressure burner (6) according toclaims 7 or 8, whereby the lamp is preferably arranged in an automotiveheadlamp unit.
 10. Method of manufacturing a gas-tight high-pressureburner (6) according to claims 7 or 8, comprising a) at least one endclosure member (9), b) at least two feed-through members (8), c) atleast one connection means (10), d) at least one sealant (5), and e) atleast one discharge vessel (1) with a coating layer (4), whereby themanufacturing method comprises the steps: i) filling said dischargevessel (1) with an ionisable filling through at least one feed-throughopening, and ii) closing said feed-through opening by arranging afeed-through (8) in said opening followed by gas-tight connecting saidfeed-through (8) to the end closure device (7) and/or to the dischargevessel (1) with connection means, whereby a gas-tight high-pressureburner (6) is obtained.